Poly N-isopropylacrylamide Gel Research Articles

Cross-Linked Polymeric Gels and Nanocomposites: New Materials and Phenomena Enabling Technological Applications

Cross-linked gels are synthesized by homo- and copolymerization of functionalized acrylamides. The gels swell in aqueous solution, and some of them (e.g., poly(N-isopropylacrylamide (PNIPAM)) also in organic solvents of low polarity (e.g., dichloromethane), making the gels amphiphilic materials. Nanocomposites can be made by dispersing nanoparticles (metallic, graphene, nanotubes, and conducting polymers) inside the gels. Additionally, true semi-interpenetrated networks of polyaniline (PANI) inside PNIPAM gels can be prepared by swelling the gel in true solutions of PANI in NMP. PNIPAM-based nanocomposites show a lower critical solution temperature (LCST) transition of the gel matrix, which can be reached by thermal heating or absorption of electromagnetic radiation (light, microwaves, radiofrequency) in the conductive nanomaterials. The characteristic properties (swelling degree and rate, LCST, solute partition, mass transport, hydrophilicity, biocompatibility) can be tuned by changing the functional groups in the copolymers and/or the other components in the nanocomposite. Mass transport and mechanical properties can be adjusted by forming materials with macro- (nanoporous and macroporous), micro- (microgels, thin films, Pickering emulsions), or nano- (nanogels, stabilized nanoparticles) sized features. The material properties are used to produce technological applications: sensors, actuators, controlled release, biological cell scaffolds and surfaces, antimicrobial, carriers of bioactive substances, and matrixes to immobilize enzymes and yeast cells.

Investigation of the luminescence, mechanical, and thermal properties of ZnO-entrapped poly(N-isopropyl acrylamide) gels

ZnO nanoparticles were synthesized in the pores that are formed inside the PNIPAm (poly(N-isopropyl acrylamide)) gels during polymerization. Different pore regions were obtained with changing the cross-linker concentrations of gels. We concluded from the SEM results that when the cross-linker concentration of the gels is increased, the average size of the pores becomes narrower. Thus, depending on the internal porosity of the gel, the size and distribution of ZnO nanoparticles were changed and accordingly it ranged in the fluorescence spectra. It is observed that fluorescence spectra of ZnO nanoparticles released from PNIPAm gel with low cross-linker concentration have only UV emission, while the nanoparticles released from PNIPAm gel with high cross-linker concentration have both the UV and the green emissions of ZnO. The size distributions of these nanoparticles were measured by Stabino-Nanoflex analyzer and it clearly indicates that the width of the size distribution decreases with increasing cross-linker concentration. To see the effect of ZnO nanoparticles on the properties of the gel, we investigated the luminescence, mechanical, and thermal properties of these ZnO/PNIPAm composites and plain gels of which the internal morphology was set by changing the cross-linker concentration. The nature of interactions between the nanoparticles and polymer strands affects both mechanical and thermal behaviors. Compared to plain gels, entrapping ZnO nanoparticles into the gels cause an increased compressive elastic modulus and strength. We proved that ZnO nanoparticles improve considerably the mechanical properties of PNIPAm gels.

Macroporous poly(N-isopropyl)acrylamide networks: formation conditions

The formation conditions of macroporous poly(N-isopropyl)acrylamide (PNIPA) networks by free-radical crosslinking copolymerization are described. The networks were prepared in water at an initial monomer concentration of 20% using N,N′-methylene(bis)acrylamide (BAAm) as the crosslinker. The synthesis parameters varied were the crosslinker concentration and the gel preparation temperature Tprep. Depending on these parameters, the variations of the swelling, mechanical and texture properties of the networks were investigated. It was shown that, after passing a critical crosslinker concentration (2–5% BAAm), the network structure changes from homogeneous to heterogeneous ones. Further increase of the crosslinker content increases both the rate of swelling and the porosity of the networks. Heterogeneous PNIPA networks consist of spherical globules called microspheres of 0.1–0.5μm in diameter aggregated to large, unshaped, discrete clusters with dimensions of a few μm. At 30% BAAm, the structure looks like cauliflowers, typical for a macroporous network. The dependence of the elastic moduli of PNIPA gels on the crosslinker content shows three different regimes depending on the structure of the networks. Increasing Tprep from 9 to 50°C at a fixed BAAm content increases both the rate of swelling and the swelling capacity of the networks, while the total porosity decreases.